(feat) Treat parens in from_str like negative sign

[brycefisher]

What It Does

Allows strings like "(12.10)" to be parsed as negative Currency values.

Why?

Because in some financial/accounting applications, currency is treated this way. Also this crate is the best Rust library on crates.io for parsing currency strings into meaningful types.

Limitations

I tried to follow the pattern I've inferred from the source code -- specifically to try not to fail on oddly formatted data instead of returning parsing errors. This means that there are tons of strings which will be successfully parsed by Currency::from_str which would look wrong to a human. See the new test cases for all the examples I could think of. Here's the most zany examples I could think of:

• ")this is a Currency struct with 0 bigint value inside" -> Currency { symbol: None, biginit::from(0) }
• "(((($1" -> Currency { symbol: Some('$'), biginit::from(-100) }

I'm not sure I feel great about this approach, and I'm very happy to change the implementation to add some strictness / sanity check :-)

[Tahler]

Thanks for your patience and the detailed PR, I'm finally getting around to this :)

It's been quite a while since I've given this repository any attention or maintenance, but hopefully it's treating you well. I added one more test case I could think of; the rest seems adequate to me.

Thanks again!